Sedimentary petrology does not study modern sediments but study lithified sediments. So a good part of it overlapped with sedimentology.
Sedimentary petrologists like to re-establish the ancient depositional environment. So they interpret the sedimentary rock "according to" what sedimentology says. As a consequence, a lot arguments in sedimentary petrology sound very much alike the contents of sedimentology.
However, there is a special part in sedimentary petrology that does not belong to sedimentology. That is the diagenesis of sedimentary rocks. Diagenesis studies all the modifictions happened to sediments after they were deposited. Even sediments lithified into sedimentary rocks, diagenesis would continue on the rock and keep modifing the rock.
Sedimentary petrology also study rock under deep burial in elevated temperature and pressure conditions. That leads to the study on the transitional area between sedimentary rock and metamorphic rock.
